Also, there have been many definitions of UTC, GMT etc over time. So to be truly correct you'd always have to mention which definition of UTC you are referring to when you use the term. And the ITU-R is debating to redefine UTC once again and remove leap seconds (http://www.ucolick.org/~sla/leapsecs/). If you are really interesting in the time stuff, here is a great resource: http://www.ucolick.org/~sla/.